Metal-Free Alternating Copolymerization of CO2 with Epoxides

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Metal-Free Alternating Copolymerization of CO2 with Epoxides: Fulfilling “Green” Synthesis and Activity Dongyue Zhang, Senthil K. Boopathi, Nikos Hadjichristidis, Yves Gnanou, and Xiaoshuang Feng J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.6b06679 • Publication Date (Web): 16 Aug 2016 Downloaded from http://pubs.acs.org on August 16, 2016

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Journal of the American Chemical Society

Metal-Free Alternating Copolymerization of CO2 with Epoxides: Fulfilling “Green” Synthesis and Activity Dongyue Zhang,† Senthil K. Boopathi, † Nikos Hadjichristidis,‡ Yves Gnanou,*† and Xiaoshuang Feng*† †

Physical Sciences and Engineering Division, King Abdullah University of Science and Technology (KAUST), Thuwal 23955, Saudi Arabia ‡

KAUST Catalysis Center, Physical Sciences and Engineering Division, King Abdullah University of Science and Technology (KAUST), Thuwal 23955, Saudi Arabia Supporting Information Placeholder ABSTRACT: Polycarbonates were successfully synthesized for the first time through the anionic copolymerization of epoxides with CO2, under metal-free conditions. Using an approach, based on the activation of epoxides by Lewis acids and of CO2 by appropriate cations, well-defined alternating copolymers made of CO2 and propylene oxide (PO) or cyclohexene oxide (CHO) were indeed obtained. Triethyl borane (TEB) was the Lewis acid chosen to activate the epoxides, and onium halides or onium alkoxides involving either ammonium, phosphonium, or phosphazenium cations were selected to initiate the copolymerization. In the case of PO, the carbonate content of the poly(propylene carbonate) (PPC) formed was in the range of 92-99% turnover numbers (TON) were close to 500; in the case of CHO perfectly alternating poly(cyclohexene carbonate) (PCHC) were obtained and TON values were close to 4000. The advantages of such a copolymerization system are manifold: i) no need for multistep catalyst/ligand synthesis as in previous works, ii) no transition metal involved in the copolymer synthesis and therefore no coloration of the samples isolated, iii) no necessity for post-synthesis purification.

Since the earlier attempts by Inoue et al. to copolymerize carbon dioxide (CO2) and epoxides,[1] significant advances have been made resulting in the development of catalysts that nowadays serve to industrially produce aliphatic polycarbonates.[2] Thanks to a precise design of the ligands bound to transition metals, these catalysts exhibit simultaneously high activity and high selectivity towards the alternating insertion of CO2 and epoxides.[2-3] Examples indeed abound of homogeneous catalysts based on transition metals such as Zn(II)[4], Cr(III),[5] Co(III)[6] and featuring precisely designed ligands that efficiently catalyze the copolymerization of aliphatic or alicyclic epoxides with CO2. In all these organo-metallic complexes involving transition metals or earth-abundant main group metals such as aluminum,[3a,7] magnesium[8] and iron,[9] the chain growth occurs by prior coordination of epoxides to the growing metal complex before their insertion at the chain ends.[4a,9a,10] Another common feature of these very active catalysts is the complexity of their ligands that are generally synthesized through multi-step synthesis. In addition, transition metal-based complexes are colored and often toxic and even if the latter are very active and used in a minute amount, a post-polymerization metal/removal step is always necessary.[6b,11]

Using an approach that totally departs from the coordinationinsertion mechanism and thus from all the work done so far we demonstrate in this study how CO2 and epoxides can be efficiently copolymerized using a metal-free process as shown in Scheme 1. From an initiator that can be an onium salt or an alkoxide associated with an organic cation the strategy used here is to bring about the alternating copolymerization of CO2 with epoxides by manipulating the reactivity of the two monomers through their activation by non-metal external activators. The activator selected for epoxides was TEB and for CO2 organic cations associated with the growing chains. An example of such non-coordinating species involving lithium salts associated with triisobutyl aluminum (TiBA) and eliciting the alternating copolymerization of CO2 with cyclohexene oxide (CHO) was recently described.[12] As illustrated in Scheme 2, the following conditions were fulfilled, allowing the alternating copolymerization to occur: 1) the rate constant of CO2 addition to the growing alkoxide chain-end (kp1,2) was much higher than the rate constant of homopolymerization of epoxide due to the activation of CO2 by Li+, thus preventing the formation of ether units; 2) because of the activation of CHO by TiBA, lithium-bound carbonate chain ends could ring-open it. The respective activation of CHO and CO2 also averted back-biting reactions (kb1,2 and kb2,1) affording perfectly linear vs cyclic selectivity. Although very simple to implement, the system comprising lithium alkoxides or lithium salts as initiator and TiBA as CHO activator suffered from its sluggishness and its overall low activity. To address this issue of low activity of the previously investigated system, we substituted cations of

Scheme 1. Anionic copolymerization of epoxides and CO2 with triethyl borane (TEB) as activator

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Table 1. Results of PO and CO2 copolymerization by different initiators in the presence of TEB (2 eq. to the initiator). All polymerizations were carried out in 50 mL autoclaves under 10 atm of CO2 at 60 ◦C for 10 hrs with equal volume of THF and PO or under conditions otherwise mentioned Entry

Initiator

Solvent

DP targeted

Yielda (%)

TONb

PPCc (mol%)

Selectivity (%)d

Mn(theo)e

Mn(GPC)f (103/PDI)

1 2 3 4h 5 6 7 8 9 10i

tBuOLi tBuONa tBuOK tBuOK BnOH+P2 BnOH+P2 BnOH+P4 BnOH+P4 NBu4Cl NBu4Cl

THF THF THF THF THF THF THF THF THF THF

50 50 50 50 50 100 50 100 50 50

8 10 87 5 82 82 81 78 85 11

--43 -41 82 40 78 42 --

-76 94 -85 88 85 85 95 --

--g 95 97 97 96 97 95 96 97 --g

-0.5 4.5 -4.2 8.4 4.1 8.0 4.3 --

-1.6/1.3 5.1/1.1 -5.8/1.1 11.0/1.1 5.7/1.1 9.5/1.1 4.8/1.1 --

11j

--

THF/PO

--

--

--

--

--

--

--

12

NBu4Cl

Bulk

50

79

39

82

95

4.0

4.7/1.1

13

NBu4Cl

Bulk

500

71

355

73

94

36.2

43.0/1.1

14

NBu4Cl

Bulk

1000

49

490

83

87

50.0

40.0/1.1

15

NBu4Cl

THF

500

66

330

94

95

34.0

25.0/1.1

16

NBu4Cl

THF

1000

46

460

92

94

45.0

50.0/1.2

17

PPNCl

THF

500

52

260

94

82

26.0

27.0/1.1

18

PPNCl

THF

1000

35

350

98

85

36.0

37.0/1.2

19

PBu4Cl

THF

1000

19

190

99

88

20.0

15.0/1.1

a

Calculated by gravimetry. b TON = mol(PO consumed) / mol(initiator). c Calculated by 1H NMR. d Calculated from IR spectra e Calculated based on the formula: Mn(theo) = 102 x (DPtarget) x (Yield%). f Determined by GPC in chloroform with polystyrene standard. g Only cyclic carbonate(propylene carbonate) was obtained. h TiBA was used instead of TEB. i No TEB was added. j Only TEB was added with same composition of TEB and PO as in entry 9. Scheme 2. Reactions involved in the anionic copolymerization of CO2 with epoxides

of larger size for lithium in association with alkoxides. We purposely selected cations capable of interacting with CO2 and activating it and at the same time bulky enough to bring about its fast copolymerization with epoxides. We reasoned that oniums and phosphazenium cations, all organic species, should fulfill the above requirements; as for the epoxide activator we opted for TEB, a Lewis acid strong enough to allow the addition of epoxides by growing carbonate anions and mild enough not to foster the homopolymerization of epoxides and the subsequent formation of ether units (see Figure S1 for its mild activation of PO).

In the copolymerization of CO2 with PO the best performing system of all tried in this study was the one composed of tetrabutyl ammonium chloride (NBu4+Cl-) and TEB. The latter system indeed exhibited the highest activity and excellent selectivity and at the same time afforded samples with the highest carbonate content (higher than 90% for entries 9, 15, 16 in Table 1). This NBu4Cl / TEB-based system thus stands out with respect to the other systems studied, exhibiting respectable TON values in the range of 500 (entry 14) and TOF values close to 50 (entry 14). Phosphonium salts also produced samples with a high content in carbonate units (>90%) but were slightly less active (TON = 350). It should be noted that both onium salts and TEB were indispensable for the formation of polycarbonates because in absence of either TEB or onium salts no copolymerization between CO2 and PO occurred (entries 10 and 11, Table 1). If the size of the cations associated with the growing anions and thus their interionic distance were the sole factor controlling the overall activity of the different systems investigated, one would have ranked the latter in the following order: P4+BnOH > PPNCl >P2+BnOH ~PBu4Cl > NBu4Cl.[13] The fact that the supposedly least active NBu4Cl /TEB system actually exhibits the highest activity (TON) indicates that besides the size of the cation both the activation of PO by TEB in (3) and that of CO2 by the cation in (2) come also into play and determine the overall activity. In the context of this investigation NBu+ cations thus appeared to simultaneously afford the highest activity (highest TONs, Table 1) and yet a high polycarbonate content. In an attempt to shed light on the interaction between these organic cations (N+, P+) and CO2 we characterized by 1H NMR the two following mixtures: PBu4Cl/CO2 and NBu4Cl/CO2. A slight upfield chemical shift is observed in the two cases for the protons in α-positions to the two

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organic cations, indicative of a small electron density donation

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formed as indicated in the MALDI-Tof spectrum (Figure S4A). Besides the perfectly alternating polycarbonate family (c) three Table 2. Results of CHO and CO2 copolymerization by different initiators in the presence of TEB (2 eq. to the initiator). All the polymerizations were carried out in 50 mL autoclaves under 10 atm of CO2 at 80 ◦C for 6 hrs with THF/CHO (1:3, v/v) or otherwise mentioned Entry

Initiator

DP targeted

Yieldsa (%)

TONb

PCHCc (mol%)

Selectivity (%)d

1 2 3

Mn(theo)e

Mn(GPC)f (103/PDI) 12.0/1.1 22.2/1.1 7.60/1.1

tBuOK 75 96 72 >99 >99 10.2 tBuOK 500 22 110 >99 >99 15.6 BnOH/ 100 75 75 >99 85 10.7 DiP-NHCg 4h BnOH+P4 250 88 220 >99 93 31.2 14.7/1.2 5 NBu4Cl 500 95 480 >99 >99 67.4 34.1/1.2 6 PPh4Cl 500 88 440 >99 >99 62.5 11.8/1.2 7 PPh4Cl 1000 92 920 >99 >99 131 19.3/1.4 8 PPh4Cl 3000 76 2280 >99 >99 324 32.4/2.1 9 PPNCl 1000 89 890 >99 >99 126 76.4/1.2 10 PPNCl 2000 74 1480 >99 94 239 29.6/1.7 11 PPNCl 4000 90 3600 >99 95 511 28.3/1.9 a Calculated by gravimetry. b TON = mol(CHO consumed) / mol(initiator). c Calculated by 1H NMR. d Calculated from IR spectra. e Calculated based on this formula: Mn(theo) = 142 x (DPtarget) x (Yield%). f Determined by GPC in chloroform with polystyrene standard. g DiP-NHC is 1,3-diisopropylimidazol-2-ylidene. h 1.0 eq. of TEB vs. initiator charged. from CO2 to them (Figure S2). For comparison purpose alkoxides associated with alkali cations (Li+, Na+, K+) were also investigated: in the case of K+, samples with high carbonate content were obtained but the overall activity was significantly lower than that of the non-metallic systems tried. One can also see (entry 4, Table 1) that stronger Lewis acids, e.g. TiBA instead of TEB as an activator of PO, afforded only oligomers with very low yield. Using less TEB ( ammonium > phosphazenium > imidazolium > potassium, which almost follows the same order as the one observed for the copolymerization of PO with CO2. When targeting high molar masses and thus increasing the feeding ratio of CHO to initiator, the linear vs cyclic selectivity slightly decreased, and the GPC traces of the samples isolated tended to become broad, or even to exhibit a bimodal distribution. A series of control experiments targeting different molar masses with PPNCl as initiator (see Table S1, Figure S7) showed that the molar masses eventually measured by GPC were lower than the expected ones, which is very likely due to the presence of water acting as transfer agent and thus affording a second population of α,ω-dihydroxyl polycarbonates besides the main population created by the initiator. Such bimodal distributions were also observed by other authors, in their attempts to synthesize samples of high molar mass from the copolymerization of CHO with CO2.[8a,10b,10d,10e] In contrast, samples of low DPs exhibited narrow and unimodal distribution in close agreement with the expected values (Table 2, Table S1, Figure S7). Such samples of low molar masses initiated respectively by potassium alkoxide and Bu4NCl were subjected to characterization by MALDI-Tof (Figure S8). In both cases only one main population with a peak to peak mass difference of 142.1 was observed, corresponding to the expected alternating structure and underpinning the incorporation of either chloride or alkoxyl moieties at the chain ends during the initiating step. The PCHC



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produced, on the other hand, showed no stereoselectivity with almost equal intensities of the peak at 153.8 ppm and those at 153.3, 153.2, 153.1 ppm (Figure S9).[5c] In this report, we have demonstrated that alternating polycarbonates could be synthesized for the first time through the anionic copolymerization of epoxides with CO2, under metal-free conditions: such a method which relies on the activation of both epoxides and CO2 by Lewis acids and appropriate organic cations respectively is facile, versatile and applicable to the copolymerization of CO2 with either PO or CHO; the TON values ( around 500 for PPC and 3600 for PCHC) are in the range of those measured for some well-known organometallic systems.[4c,5a,6g,10d] The polycarbonates synthesized under such metal-free conditions do not exhibit coloration, and are thus free of metal residues; they could well be used without further purification in areas such as food package, coating, biological and electronic fields. Their respectable TON values combined with the easy availability of both initiator and activator make polycarbonates produced under these conditions very attractive for various applications. Metal-free syntheses of polycarbonates with other epoxides than PO or CHO and structures are currently investigating in our group. ASSOCIATED CONTENT Supporting Information Experimental details and additional characterization data, including Table S1 and Figures S1-S9. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author [email protected], [email protected] Notes The authors declare no competing financial interests. REFERENCES [1] Inoue, S.; Koinuma, H.; Tsuruta, T. Polym. Lett. 1969, 7, 287-292. [2] Trott, G.; Saini, P.; Williams, C. Phil. Trans. R. Soc. A 2016, 374, 20150085. [3] a). Ikpo, N.; Flogeras, J. C.; Kerton, F. M. Dalton Trans. 2013, 42, 8998-9006; b). Darensbourg, D. J.; Wilson, S. J. Green Chem. 2012, 14, 2665; c). Kember, M. R.; Buchard, A.; Williams, C. K. Chem Commun (Camb) 2011, 47, 141-163; d). Darensbourg, D. J. Chem. Rev. 2007, 107, 2388-2410; e). Coates, G. W.; Moore, D. R. Angew. Chem. Int. Ed. 2004, 43, 6618-6639; f). Sugimoto, H.; Inoue, S. J. Polym. Sci., Part A: Polym. Chem. 2004, 42, 5561-5573; g). Lu, X.-B.; Ren, W.-M.; Wu, G.-P. Acc. Chem. Res. 2012, 45, 1721-1735.

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[4] a). Kissling, S.; Lehenmeier, M. W.; Altenbuchner, P. T.; Kronast, A.; Reiter, M.; Deglmann, P.; Seemann, U. B.; Rieger, B. Chem. Commun. 2015, 51, 4579-4582; b). Lee, B. Y.; Kwon, H. Y.; Lee, S. Y.; Na, S. J.; Han, S. I.; Yun, H.; Lee, H.; Park, Y. W. J. Am. Chem. Soc. 2005, 127, 3031-3037; c). Allen, S. D.; Moore, D. R.; Lobkovsky, E. B.; Coates, G. W. J. Am. Chem. Soc. 2002, 124, 14284-14285; d). Cheng, M.; Moore, D. R.; Reczek, J. J.; Chamberlain, B. M.; Lobkovsky, E. B.; Coates, G. W. J. Am. Chem. Soc. 2001, 123, 8738-8749; e). Cheng, M.; Lobkovsky, E. B.; Coates, G. W. J. Am. Chem. Soc. 1998, 120, 11018-11019. [5] a). Darensbourg, D. J.; Ulusoy, M.; Karroonnirum, O.; Poland, R. R.; Reibenspies, J. H.; Çetinkaya, B. Macromolecules 2009, 42, 6992-6998; b). Darensbourg, D. J.; Yarbrough, J. C.; Ortiz, C.; Fang, C. C. J. Am. Chem. Soc. 2003, 125, 7586-7591; c). Darensbourg, D. J.; Yarbrough, J. C. J. Am. Chem. Soc. 2002, 124, 6335-6342. [6] a). Ren, W. M.; Liu, Z. W.; Wen, Y. Q.; Zhang, R.; Lu, X. B. J. Am. Chem. Soc. 2009, 131, 11509-11518; b). Sujith, S.; Min, J. K.; Seong, J. E.; Na, S. J.; Lee, B. Y. Angew. Chem. Int. Ed. 2008, 47, 7306-7309; c). Noh, E. K.; Na, S. J.; Sujith, S.; Kim, S. W.; Lee, B. Y. J. Am. Chem. Soc. 2007, 129, 8082-8083; d). Nakano, K.; Kamada, T.; Nozaki, K. Angew. Chem. Int. Ed. 2006, 45, 7274-7277; e). Cohen, C. T.; Chu, T.; Coates, G. W. J. Am. Chem. Soc. 2005, 127, 10869-10878; f). Lu, X. B.; Wang, Y. Angew. Chem. Int. Ed. 2004, 43, 3574-3577; g). Qin, Z.; Thomas, C. M.; Lee, S.; Coates, G. W. Angew. Chem. Int. Ed. 2003, 42, 5484-5487. [7] a). Sheng, X.; Wu, W.; Qin, Y.; Wang, X.; Wang, F. Polym. Chem. 2015, 6, 4719-4724; b). Wu, W.; Sheng, X.; Qin, Y.; Qiao, L.; Miao, Y.; Wang, X.; Wang, F. J. Polym. Sci., Part A: Polym. Chem. 2014, 52, 23462355. [8] a). Kember, M. R.; Williams, C. K. J. Am. Chem. Soc. 2012, 134, 15676-15679; b). Xiao, Y.; Wang, Z.; Ding, K. Macromolecules 2006, 39, 128-137. [9] a). Buchard, A.; Kember, M. R.; Sandeman, K. G.; Williams, C. K. Chem. Commun. 2011, 47, 212-214; b). Nakano, K.; Kobayashi, K.; Ohkawara, T.; Imoto, H.; Nozaki, K. J. Am. Chem. Soc. 2013, 135, 84568459. [10] a). Ohkawara, T.; Suzuki, K.; Nakano, K.; Mori, S.; Nozaki, K. J. Am. Chem. Soc. 2014, 136, 10728-10735; b). Moore, D. R.; Cheng, M.; Lobkovsky, E. B.; Coates, G. W. J. Am. Chem. Soc. 2003, 125, 1191111924; c). Saini, P. K.; Romain, C.; Williams, C. K. Chem Commun (Camb) 2014, 50, 4164-4167; d). Kember, M. R.; Knight, P. D.; Reung, P. T. R.; Williams, C. K. Angew. Chem. Int. Ed. 2009, 48, 931-933; e). Lee, B. Y.; Kwon, H. Y.; Lee, S. Y.; Na, S. J.; Han, S.-i.; Yun, H.; Lee, H.; Park, Y.-W. J. Am. Chem. Soc. 2005, 127, 3031-3037; f). Garden, J. A.; Saini, P. K.; Williams, C. K. J. Am. Chem. Soc. 2015, 137, 15078-15081. [11] a). Hongfa, C.; Tian, J.; Andreatta, J.; Darensbourg, D. J.; Bergbreiter, D. E. Chem. Commun. 2008, 975-977; b). Hauenstein, O.; Reiter, M.; Agarwal, S.; Rieger, B.; Greiner, A. Green Chem. 2016, 18, 760-770; c). Bahramian, B.; Ma, Y.; Rohanizadeh, R.; Chrzanowski, W.; Dehghani, F. Green Chem. 2016, 10.1039/c5gc01687h. [12] Zhang, D.; Zhang, H.; Hadjichristidis, N.; Gnanou, Y.; Feng, X. Macromolecules 2016, 49, 2484-2492. [13] Labbé, A.; Carlotti, S.; Billouard, C.; Desbois, P.; Deffieux, A. Macromolecules 2007, 40, 7842-7847. [14] Chisholm, M. H.; Navarro-Llobet, D.; Zhou, Z. Macromolecules 2002, 35, 6494-6504.


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Metal-Free Copolymerization of CO2 with Epoxides: Towards a “Greener” Synthesis of Aliphatic Polycarbonates Dongyue Zhang,† Senthil K. Boopathi, † Nikos Hadjichristidis,‡ Yves Gnanou,*,† and Xiaoshuang Feng*


Metal-Free Alternating Copolymerization of CO2 with Epoxides

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